Servo-mechanism for controlling the movements of an electrode in an electro-erosion machine



June 20, 1961 N. MIRONOFF 2,989,616

SERVOMECHANISM FOR CONTROLLING THE MOVEMENTS OF AN ELECTRODE IN AN ELECTRO-EROSION MACHINE Filed Aug. 11, 1958 B S JERVd United States Patent 2,989,616 SERVO-MECHANISM FOR CONTROLLING THE MOVEMENTS OF AN ELECTRODE IN AN ELECTRO-EROSION MACHINE Nicolas Mironoif, Geneva, Switzerland, assignor to Atelier-s des 'Charmilles S.A., Geneva, Switzerland, a corporation of Switzerland Filed Aug. 11, 1958, Ser- No. 754,294 Claims priority, application Switzerland Aug. 26, 1957 Claims. (Cl. 219-69) Servo-mechanisms are known for controlling the movements of an electrode in an electro-erosion machine, comprising a diiferential piston sliding in a cylinder for the control of the movements of the electrode, the movements of said piston producing variations of unequal volume and of inverse direction in two Working chambers; a hydraulic circuit comprises a pump delivering a liquid under constant pressure, a first passage conducting said liquid into the working chamber which is subjected to the smallest possible variations of volume in proportion to the movements of the differential piston, and a second passage for causing the first passage to communicate with the other Working chamber, said second passage having a connection of adjustable cross-section to the escape of the liquid under pressure; said connection is controlled by a device sensitive to the electrical conditions of the machine in operation.

The servo-mechanisms of this type are advantageous, as they react very rapidly. On the contrary, when said servomechanisms are not in operation and the piston moves under the action of its proper weight or an external action, a depression is produced in one of the working chambers, which promotes an entry of air into said chamber. Such an entry of air constitutes a major inconvenience by reason of the compressibility of the air which causes a lack of precision of the servo-mechanisms. Further, the manual movement of the differential piston is difficult, as the cross-section of the constricted passage is very small, as is also that of the passage of adjustable cross-section, so that a considerable force has to be applied to the differential piston so as to provoke the circulation of liquid in the said passages.

The present invention has for its object to remedy these inconveniences, by reason of the fact that the servo-mechanism comprises means permitting the connection to exhaust of at least one of the two working chambers in such a manner as to facilitate the manual movement of the differential piston when the servo-mechanism is out of use and to avoid entries of air.

One form of construction of the servo-mechanism is shown diagrammatically and by way of example in the accompanying drawings, wherein:

FIG. 1 is a diagram of an electro-erosion machine.

:FIG. 2 shows in section the servo-mechanism of said mechanism.

The basic diagram of an electro-erosion machine, shown in FIG. 1, shows that the machine comprises an electric generator A of which the terminals a and b are connected, on the one hand, to an electrode E and, on the other hand, to a part P to be machined which are to be plunged into a dielectric liquid. Said generator supplies the energy necessary for producing a series of discharges by sparks between the part P and the electrode E, each spark removing a little metal to the part to be machined. A servo-mechanism S is provided for causing the electrode E to advance in the direction of the part P in proportion to the machining of said latter. The control of the servo-mechanism is ensured electrically by a device B which measures the voltage between the electrode E and the part P and supplies to the servo-mechanism a control current depending on. the said voltage.

The servo-mechanism which is shown in detail in FIG.

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2 comprises a differential piston =1 secured to a rod 2 having different diameters on each side of said latter. The electrode E is secured by a device, not shown, to the lower part of the rod 2. The differential piston slides in a cylinder 3 and produces unequal variations in volume and of inverse directions in two working chambers 4, 5, respectively. The hydraulic circuit comprises a reservoir 6 feeding a pump 7 adapted to deliver the control liquid under a constant pressure. Said liquid is passed through a first passage 8 into the working chamber 5. A second passage 9 causes the first passage 8 to communicate with the other working chamber 4. Said second passage has a restricted passage 10 and, at the upfiow of said latter, a passage 11 of adjustable crosssection permitting the liquid to be connected to outflow. The liquid passing out through the passage 11 passes into a reservoir 6a, then passes out through an overflow 6b into the reservoir 6. The cross-section of said passage 11 is adjusted by a ball 12 which is more or less withdrawn from its seat provided in a part 13, by means of a finger 14 of a device 15 responsive to the electrical conditions of operation of the machine. Said device 15 is not described in detail, as it does not form part of the invention.

The pipe 9 also has a passage of which the section is adjustable by means of a screw 16 which may be reached from the exterior by a bore 17. This pipe may be placed into communication with the reservoir 6a, by means of a passage capable of being closed by an automatic valve 18 which is subjected to the action of a spring 19 tending to open it. Further, the passage 8 may also communicate with the reservoir 6a in the bottom of which is provided a second automatic valve 20 subjected to the action of a spring 21 tending to open it.

For ensuring a constant pressure in the chamber 5, the passage 8 has a discharge valve comprising a valve 22 subjected to the action of a spring 23 which tends to apply it against a seat 24. A screwed plug 25 is provided for the mounting and removal of the discharge valve. When the liquid supplied into the passage 8 exceeds a determined pressure, it displaces the valve 22 against the action of the spring 23 and can escape for returning into the reservoir 6 through the passage 6b. The discharge valve is placed as closely as possible to the constricted passage 10 and to the working chamber 5 so as to avoid variations in pressure due to the inertia of the liquid contained in the passage 8 during the variations in supply in said passage.

Each end of the cylinder 3 is closed by a part 26, 27, respectively, each having a bore 28, 29, respectively, provided with fluidtight linings and in each of which slides a portion of the rod 2. Each of the parts 26 and 27 also has an internal space 30 and 31 communicating with the reservoir 6a for the return of liquid by a passage 32. It will be seen that two spaces 30 and 31 are placed in communication with each other by a passage 33 provided in the wall of the cylinder 3. The reservoir 6a is at a higher level than the spaces 30 and 31, so that these latter are always filled with liquid under a pressure determined by the height of the level in the reservoir 6a. The rod 2 fits with clearance in bores 34, 35, respectively, provided in each of the said parts and located between each of the said spaces 30 and 31 and the adjacent working chamber 4, 5 respectively.

The operation of the device described is as follows:

The pump 7 places the liquid under pressure and this pressure is transmitted into the chamber 5. The liquid supplied by the pump passes through the restricted passage 10, which produces a loss of pressure, so that the pressure in the passage 9 and in the chamber 4 is lower than that in the passage 8 and the chamber 5. The liquid supplied by the pump then passes through the passage 11 3 of adjustable cross-section, in which passage a new drop in pressure is produced. By reason of the pressures in the passages 8 and 9, the valves 18 and 20 are closed against the action of their springs 19 and 21, respectively. It is clear that the pressure in the chamber 4 is deter-- mined by the free cross-section of the adjustable passage 11. When, for a given opening of said passage 11, the ratio of the pressures in the chambers 4 and is equal to the inverse of the ratio of the active surfaces of the differential piston in said chambers 4 and 5, a stage of equilibrium is present and the piston does not move, if the weight of the rod 2 and of the electrode E is neglected. If at this moment, the ball 12 is withdrawn farther from its seat so as to provide a greater opening 11, the pressure in the chamber 4 decreases, whereby an upward movement of the piston 1 is produced. A closing of the passage 11 by the ball 12 produces a movement in the opposite direction. By acting on the screw 16, it is possible to modify the resistance of the flow in the passage 9 and thus vary the rapidity of reaction of the servo-mechanism.

When material has been removed from the part P, the distance separating said part P from the electrode E increases, thus producing a consecutive increase of the disruptive voltage and of the average between these parts. The device B reacts by transmitting to the device 15 a current producing a downward movement of the finger 14. This results in the diminution of the passage 11 and an increase of the pressure in the chamber 4, thus producing a movement of the rod 2 and therefore of the electrode E in the direction of the part P. This reduction of the distance between the electrode and the part produces a diminution of the average voltage between the electrode E and the part P, so that the finger 14 resumes its initial position and the pressure in the chamber 4 resumes the value for which the diflerential piston 1 is not subjected to movement. It will be readily understood that when the distance between the electrode E and the part P commences to decrease, for example by reason of the accumulation of conductive dust, the servo-mechanism reacts in a contrary direction and produces a withdrawal of the electrode E relatively to the part P.

During the operation of the servo-mechanism, liquid flows from the chambers 4 and 5, respectively, towards the spaces 30 and 31 provided in parts 26 and 27. However, this flow is very weak, as the clearance provided between the rod 2 and the bores 34 and 35 is very small, of the order of a few hundredths of a millimeter. However, as the free section between said bores and the rod 2 remains constant, the slight leakages of said chambers 4 and 5 do not in any way affect the functioning of the servo-mechanism. On the contrary, this arrangement is advantageous as the fluidtight joints are not subjected to the relatively high pressure obtaining in the chambers 4 and 5, but solely to the very low pressure exerted by the liquid contained in the reservoir 6a.

When the machining is terminated or simply interrupted, the pump 7 is stopped, and the pressure of the liquid in the passages 8 and 9 and the chambers 4 and 5, dissipated through the bores 34 and 35 to the reservoir 6a. The automatic valves 18 and 20 then open under the action of their springs 19 and 21 and enable the chambers 4 and 5 to communicate directly with the reservoir (in. It is thus easy to move the rod 2 by hand, as the communication of the chambers 4 and 5 with the reservoir 6a is effected by passages having very Weak losses of pressure. The arrangement also avoids entry of air into the chambers 4 and 5 when the rod 2 is moved manually.

At the moment at which the pump 7 is again put into operation, the liquid under pressure causes the closing of the valves 18 and 20, so that the servo-mechanism is ready to function.

It will be understood that numerous modifications may be made in construction, and the automatic valves may be located at ditferent places. In particular, one of the 2,989,616 g 0 r r valves may be located in the differential piston so as to cause the two working chambers to communicate directly with one another when the pressure is practically zero. The latter should be actuated so as to close automatically when the pressure in the chamber 5 becomes higher than that in the chamber 4. The second valve may be provided either on the passage 8, or on the passage 9, so as to cause one of said passages to communicate with the reservoir 6a. In a further modification, it is also possible to replace the automatic valves by valves with an electromagnetic control, of which the closure is controlled at the same time as the setting in operation of the pump 7.

In a more simple form of construction, the closing parts of the cylinder 3 may have only one bore in which the rod 2 slides, the spaces and 31 being then omitted. In this case, the fluidtight linings are subjected to much higher pressures, but the introduction of air during the manual movements of the rod 2 can no longer be produced by reason of the satisfactory communication of the chambers 4 and 5 with the reservoir 6a and of the higher level of this.

It is also possible to replace the valves 18 and 20 by other elements enabling the chambers 4 and 5 to be placed to exhaust, for example by means of a slide distributor or by valves controlled automatically or manually. According to a simpler form of construction, it is possible to be satisfied with means enabling only one of the working chambers 4 or 5 to be placed to exhaust. When the passage of' adjustable cross-section is controlled by the device 15 in such a manner that on the stoppage of the servo-mechanism, said passage has a relatively large free cross-section, means enabling the chamber 5 to be connected to exhaust may be suflicient.

I claim:

1. A servo-mechanism for controlling the movements of an electrode in an electro-erosion machine with respect to a workpiece, comprising a hydraulic cylinder, a slidable double acting differential piston in said cylinder providing therein two hydraulic chambers, the effective area of the piston subjected to fluid pressure in the first of said chambers being smaller than the effective area of the piston in the second chamber so that greater displacement of the piston is produced by application of a unit volume of fluid pressure to said effective piston area in said first chamber than by application of a unit volume of fluid pressure to said effective piston area in said second chamher, said piston actuating the electrode, a source of fiuid pressure, a supply line connecting said source with said first chamber, a branch line connecting said supply line with the second chamber, a return circuit for the hydraulic fluid to said source, means reducing the hydraulic pressure of the fluid entering through the branch line into the second chamber below the pressure of the fluid entering the first chamber, a passage connecting said branch line to the return circuit, adjustable valve means in said passage, means responsive to the changes of the voltage across said electrode and workpiece, said voltage responsive means controlling said valve means, passages connecting said supply line and said branch line, respectively, to said return circuit, and spring actuated valves in said passages, said valves being closed when said source of fluid pressure is operating and being fully opened and establishing simultaneous connections of said chambers to said return circuit when said source of fluid pressure is not operating, thereby relieving the hydraulic pressure in both said chambers and allowing manual displacement of said piston without admission of air into the hydraulic system.

2. The servo-mechanism claimed in claim 1, wherein said adjustable valve means in the passage connecting the branch line and the return circuit comprise a floating ball valve and a vertically displaceable finger controlled by said voltage responsive means, said finger controlling the position of said ball.

3. The servo-mechanism as claimed in claim 1 including a reservoir in said return circuit, said voltage responsive means being disposed in said reservoir and said connecting passages opening into said reservoir.

4. The servo-mechanism as claimed in claim 1, comprising members closing the ends of said cylinder, an internal cavity in each of said members communicating with said reservoir, said reservoir being placed at a higher level than said cavities, two registering bores in each of said members, and a piston rod at each side of said piston, said rods having different diameters and passing through said cavities and bores.

5. The servo-mechanism claimed in claim 4, wherein said rods pass with clearance through said bores provided between said cavities and hydraulic chambers.

References Cited in the file of this patent UNITED STATES PATENTS 400,323 Geyer Mar. 26, 1889 2,105,473 Dean Jan. 18, 1938 2,356,597 Kronenberger Aug. 22, 1944 2,479,063 Forsythe Aug. 16, 1949 2,572,385 Price Oct. 23, 1951 2,696,403 Baugh Dec. 7, 1954 2,751,752 Metcalf June 26, 1956 

